1. Technical Field
This invention relates to the technology for moving molten metal from a heated molten furnace to a die cavity by use of low pressure in the furnace.
2. Discussion of the Prior Art
Casting systems that deliver molten metal against the force of gravity generally fall into two categories: pneumatic or electromagnetic pumping. Such systems are particularly useful for casting complex or thin-sectioned articles as the metal will be delivered slowly and tranquilly. The pneumatic type is of importance because of its better reliability, ease of maintenance and minimal experimentation. The metal is pressurized in the furnace with air or other gases to develop a differential pressure between the furnace and mold, which differential pressure forces the metal from the furnace into the mold. Such pneumatic systems are difficult to precisely control because (i) any changes, in the metal flow into the mold, are countered by the momentum of the remaining pressurized supply of the entire furnace, (ii) by the necessity of returning the unused metal supply to the furnace which retransfer may lead to additional oxidation or solution of contaminated gases, and (iii) by the need for added heating to keep the retransferred metal in a molten condition.
Low pressure molding of metals, such as aluminum alloys for automotive components, including heads and blocks, has advanced to the use of a cast iron gatebox between the holding furnace and the die assembly or mold. A single cast iron tube extends from the bottom of the gatebox into the molten metal within the furnace. Radiant heaters may be located above the mold and around the gate box and tube to maintain the metal molten at an elevated temperature. When the mold is in a sealed metal receiving position, over the gatebox, low pressure on the metal gradually forces the molten metal to rise in the tube, fill the reservoir of the gatebox and thence flood the inlets to the base of the die cavity. Radiant or other heaters are located above the mold assembly and around the gatebox and tube to maintain the metal molten at an elevated temperature. Upon completion of metal filling of the mold, pressure is relieved in the furnace and excess molten metal in the gatebox recedes back into the furnace. The rising and receding of the molten metal contributes to the formation of small minute oxide particles in the molten metal, which oxides will eventually be present in the casting.
Multiple tubes or stalks have been deployed by the prior art but only as a direct fluid communication between the mold and furnace; only a single tube has been deployed between a gate box and furnace to the knowledge of applicants.